The invention relates to a process for reducing the noise of a video signal, in particular, when reproducing a video signal stored on a video recorder.
It is known for the noise reduction of video signals to use so-called sigma filters (cf., for example, JongSen Lee "Digital Image Smoothing and the Sigma Filter" in Computer Vision, Graphics, and Image Processing 24, 255 to 269 (1983)). The functional principle of sigma filters is based on replacing a signal value of a video image belonging to a picture element with a determined signal value which is formed by averaging the neighboring signal values, the intensity of which lies within a fixed standard deviation range (sigma range) around the intensity value of the output signal value. The basic idea of this filtering is that noise interferences occur approximately with a Gaussian distribution, so that effective noise suppression is possible within the standard deviation range. If there is a greater difference in intensity between a neighboring signal value and the output signal value, there is a high probability that this difference in intensity is determined not by noise but by some other content of the picture signal. This signal value should, therefore, not be included in the averaging.
Therefore, the mean deviation of the signal values must first be established for each defined window. Limit values are determined according to the established mean deviation. The intensity values lying within these limit values are included in the averaging.
Such a process is also known from Jung, Kim "Adaptive Image Restoration of Sigma Filter Using Local Statistics and Human Visual Characteristics", Electronics Letters, 1988, Vol. 24, No. 4, pages 201 to 202. For adaptation of the limit values for the sigma range, these values are fixed proportionally with respect to the square root of the signal variance (.sigma..sup.2) for each pixel.